1. Field of the Invention
This invention relates generally to photolithography techniques for patterning semiconductor devices, and, more particularly, to a method for controlling photoresist removal processes.
2. Description of the Related Art
Conventionally, semiconductor devices are patterned using photolithographic processes. A base material, such as a substrate material, a metal, an insulator, etc., is coated with a light sensitive material, referred to as a photoresist material. The photoresist is generally a composition that is sensitive to active rays of light, such as ultraviolet rays, X-rays or electron rays. The photoresist is deposited on the base material to selectively protect non-process portions of the substrate. Light is then selectively directed onto the photoresist film through a photomask to form photoresist patterns on the base material. The photoresist is then developed to remove either the exposed photoresist or the unexposed photoresist.
There are generally two types of photoresist, namely positive type and negative type. The positive photoresist is of such a type that the exposed portion dissolves in the developer, while the unexposed portion does not dissolve therein, and the negative photoresist is of the opposite type. Certain photoresist materials do not complete the transition from being soluble to being insoluble in the developer based solely on the exposure to light. These photoresist materials, referred to as chemically-assisted photoresists, are subjected to a post exposure bake process to complete the chemical reaction to transition from soluble to insoluble (i.e., for a positive resist).
The process of using a chemically-assisted photoresist is described in greater detail in reference to FIGS. 1A through 1D. FIG. 1A shows a wafer 10 including a base material 12 with a photoresist layer 14 deposited thereon. In FIG. 1B, the photoresist layer 14 is exposed to a light source through a photomask (not shown) to define exposed regions 16. Exposure to the light causes hydrogen free radicals to form in the exposed regions 16, which are on the surface of the photoresist layer 14. In FIG. 1C, the wafer 10 is subjected to a post exposure bake to complete the solubility transition chemical reaction and form baked regions 18. During the post exposure bake, the free radicals diffuse downward and react with the photoresist 14 beneath the exposed regions 16. Typically, for a deep UV photoresist layer 14, the post exposure bake time is about 60-90 seconds. As shown in FIG. 1D, a developer may then be applied to remove the remaining photoresist 14 (i.e., for a negative resist--shown in FIG. 1D) or to remove the baked portions 18 (i.e., for a positive resist--not shown).
Accurate control of the developing process, also referred to as the photoresist removal, is important for preventing defects in the wafer. If the photoresist removal time is too short, remnants of the photoresist layer will be present on the wafer, interfering with subsequent processing steps. Typically, a minimum removal time is programmed into the recipe of the developer, however, variations in the photoresist, developer, photoresist layer thickness, etc., may result in different photoresist removal rates for various wafers in the same or different lots. Accordingly, a minimum removal time does not always ensure that all of the photoresist is removed.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.